1. Field
The following description relates to a light sensing circuit, a touch panel including the light sensing circuit, and a method of driving the light sensing circuit.
2. Description of the Related Art
A mouse or keyboard is generally used to input signals to a computer. Also, a remote control is used to select specific functions in a digital television. However, users who are unfamiliar with such devices may experience difficulties in properly using them.
A touch panel or a touch screen is a new input device concept which may be used by less experienced users. In this case, when a user wants to input signals to a computer or digital television, the user directly touches the touch panel by using a finger or a pen.
Touch panels may be classified as a capacitive sensing type, a resistive touching type, an infrared beam sensing type, a surface acoustic wave type, an integral strain gauge type, a Piezo effect type, or a light sensing type, according to a method of recognizing input signals.
The light sensing type touch panel comprises a display including a photodiode, where a finger's touch is recognized by sensing a current generated by light incident on the photodiode. The photodiode may be manufactured when manufacturing a driving circuit of the display which may be, for example, a liquid crystal display (LCD) or an organic light emitting diode (OLED) display. Also, as in the case of the resistive touching type touch panel or the capacitive sensing type touch panel, a separate layer may not be needed outside of the display, and thus the thickness of the light sensing type touch panel may be reduced.
However, a current value of the photodiode used in the light sensing type touch panel is very low. For example, there may only be a slight change in current according to an applied voltage value, and a photodiode area and the current generated in the photodiode is small, on the order of about a few to several tens of pA.
In addition, the light sensing type touch panel includes a plurality of pixel circuits for displaying gradation and a plurality of light sensor circuits for sensing touch of pixels. Thin film transistors (TFTs) are used to amplify a current of the photodiode in the plurality of light sensor circuits. As threshold voltages Vth of driving transistors for amplifying a current vary due to fabrication and/or manufacturing variables, causing output values of each light sensor circuit to vary, the accuracy of touch determination or touch location determination is reduced or compromised.